1. Field of the Technology
The present technology relates to an optical distance measuring sensor and an electronic device, and particularly to an optical distance measuring sensor for measuring a distance to a target object located within a predetermined range as well as an electronic device including the sensor.
2. Description of the Background Art
There have been known optical distance measuring sensors for measuring a distance to a target object located within in a predetermined range by a triangulation method. FIG. 13 is a cross section showing a conventional optical distance measuring sensor.
Referring to FIG. 13, the conventional optical distance measuring sensor includes a light emitting element 100, a light receiving element 200, an integrated circuit unit 200A that drives light emitting element 100 and processes signals provided from light receiving element 200, transparent resin units 300 (300A and 300B) sealing light emitting element 100 and light receiving element 200, respectively, lenses 400 (400A and 400B) on a light emitting side and a light receiving side, a light-blocking resin unit 500 holding light emitting element 100, light receiving element 200 and transparent resin unit 300, a lead frame 600 and a lens casing 700.
In the distance measuring sensor shown in FIG. 13, light receiving element 200 is provided with only one PSD (Position Sensitive Detector). The PSD has a resistance value that is uniform and constant throughout the PSD. Assuming that I1 and I2 indicate signals (current values) obtained from two terminals of light receiving element 200, respectively, distance measurement is performed by obtaining an output value corresponding to a distance from (I1/(I1+I2)).
The optical distance measuring sensor described above has been disclosed, e.g., in Japanese Patent Laying-Open Nos. 09-318315 and 2003-287420.
Japanese Patent Laying-Open No. 2002-195807 has disclosed a structure that uses a CMOS image sensor as a position sensing element.
In the optical distance measuring sensor shown in FIG. 13, the PSD (Position Sensitive Detector) used as the element on the light receiving side includes only one light receiving unit, and the resistance value of the light receiving unit is uniform and constant throughout the light receiving unit. Therefore, an accuracy of distance measurement is high when a distance to the object is short, but the accuracy of distance measurement is low when the distance to the object is long. The cause of this problem will be described below with reference to FIGS. 14 and 15.
Referring to FIG. 14, in an optical distance measuring sensor that senses a distance (d) to an object by the triangulation method, the distance (d) to the object is inversely proportional to an output (X) of a light receiving unit as shown in FIG. 15, and the changes in output caused by variations in distance are large when the distance to the object is short, and are small when the distance to the object is long. Therefore, in a range where the distance (d) to the object is long, large variations do not occur in output (X) of the light receiving unit even when a predetermined amount of variations occur in distance (d).
Conversely, the accuracy can be increased by increasing a center distance (A) between centers of lenses on the light emitting side and the light receiving side, or a distance between the lens on the light receiving side and the light receiving element, i.e., a focal distance (f) of the lens on the light receiving side. However, the increase in such distances (A) and (f) results in increase in size of the optical distance measuring sensor.
In the conventional distance measuring sensor, the accuracy of the distance measurement is low when the distance (d) to the object is long. Therefore, only a practical measurable distance range in which a ratio between the maximum and minimum measurable distances is substantially eight or less has been available.
Japanese Patent Laying-Open No. 2002-195807 has disclosed the structure that employs the CMOS image sensor as the position sensing element. In this document, however, a light emitting element and a light receiving element are not located on the same plane, and each element employs an independent package. Further, a unit of processing a signal provided from the CMOS image sensor as well as a drive circuit unit of the light emitting element are arranged outside a CMOS image sensor chip. Consequently, the structure in Japanese Patent Laying-Open No. 2002-195807 suffers from a problem that the distance measuring sensor has large sizes as well as problems that production steps are large in number, operations in various steps are complicated when high accuracies are required, and a production cost is high.